Download The use of radiation is improving the biological control of insect pests.

To Kill a Pest
by Jorge Hendrichs and
Alan Robinson
The use of radiation is improving the biological
control of insect pests.
T
A giant ichneumon wasp
adult boring the surface
of fir trunk infested with
wood wasp larvae.
(Photo: Boris Hrasovec,
Faculty of Forestry,
Bugwood.org)
he IAEA’s support to Member States in
the field of insect pest control has mainly
focused on the Sterile Insect Technique (SIT),
which is a type of insect birth control, where mass
reared and systematically released sterile males
of the target pest insect mate with wild females
in the field, thereby interfering in an environmentfriendly way with the reproduction of the pest
population. This approach effectively reduces
the use of insecticides and has been successfully
used to manage, and in some cases eradicate,
populations of major pest insects. Nevertheless,
there are other areas where Member States can
benefit from radiation in the field of entomology.
One of these is biological control.
34 | IAEA Bulletin 51-1 | September 2009
What is Biological Control?
Despite centuries of technological development,
insect pests continue to exact a very high toll on
agricultural production and human health. A wellestablished, successful approach to this problem is
the use of natural enemies, called biological control
agents, to manage pest populations. The biological
control agent can be a predator, a parasitoid, a bacterium, a fungus or a virus. In this article we will concentrate on predators, which eat the pest (prey), and
parasitoids, which parasitize the pest (host) by stinging and thereby laying eggs into it.
When insects escape their native natural enemies,
either because they invade new countries leaving
Animal Kingdom | To Kill a Pest
behind their biological control agents or as a result
of the disturbance of these natural enemies, they
become pests. As shown in Box 1, if appropriately
applied, biological control offers one of the most
promising, environmentally-sound, and sustainable tools for control of such insect pests. However,
there are many constraints to the expansion of biological control programmes, related to the production, shipment and release of biological control
agents. The commercial and public biological control industry is growing but still represents less than
3% of pest control sales. Regulatory, technical and
other constraints have kept the market share relatively small. The challenges include the high cost
of production, adequate quality control and assurance, trade barriers and regulations that complicate
shipping.
There are several ways in which nuclear techniques
can address these constraints and the Joint FAO/
IAEA Programme of Nuclear Techniques in Food
and Agriculture has now completed a Coordinated
Research Project on this topic with the participation
of 18 research teams from 15 countries.
Regulating Biological Control
Agents
Lack of international harmonization among countries and availability of enabling regulations is
probably the most important barrier to the wider
implementation of biological control, ‘gatekeeper’
regulations place barriers in the way of efficient
introduction and application of biological control
agents. However, the Secretariat of the International
Plant Protection Convention, of the Food and
Agriculture Organization of the United Nations
(FAO) has published a revised International Standard
for Phytosanitary Measures (ISPM) on ‘Guidelines for
the export, shipment, import, and release of biological control agents and other beneficial organisms’,
which should help to solve some of these problems and increase trans-boundary trade in biological control agents.
Safeguarding Biodiversity
At times, the biological control agent chosen to
be released is an exotic species to the environment. One of the key concerns in this approach is
the question of specificity of the biological control agent in its new ecosystem, i.e. will the introduced biological control agent remain associated
with the pest or will it expand its range to impact
on other species in the ecosystem and so affect bio-
Biological Control of the
Cassava Mealybug
T
his insect became a devastating pest of cassava in Sub-Saharan Africa following its introduction into Congo from South America in the
1970s and its rapid spread to the rest of the subcontinent. Cassava is the prime source of carbohydrates, proteins and vitamins for 200 million
Africans. A natural parasitoid (insert image) of the
cassava mealybug was identified in Paraguay, the
origin of the pest in South America, and transferred to the International Institute for Tropical
Agriculture, in Ibadan, Nigeria where it was mass
reared. It was released from over 150 sites in the
region, became established and brought the
pest under control in 95% of all fields.
Screening Exotic
Biological Control Agents
to Safeguard Biodiversity
♦ In the USA the field evaluation of an exotic
herbivore in sterilized form, for the eventual
biological control of the Brazilian pepper tree,
a major weed that has been accidentally introduced.
♦ Also assessment of irradiated cactus moths,
a pest of different native cactus plant in some
locations, and a biological control agent of introduced cactus weeds in others, to confirm under
natural conditions oviposition preferences to
predict the host range, the ability of larvae to survive and cause damage on related native plants,
as well as to study possible interactions with natural enemies.
diversity or even attack beneficial species or commercial crops? There are numerous examples where
introduced biological control agents have ‘jumped
species’. Since such mistakes are permanent in time
and space, the feasibility of such introductions need
therefore to be carefully assessed before the introduction of any biological control agent is undertaken. One of the best-known of these mistakes is
the cane toad in Australia, which was introduced to
control pest insects in sugar cane, however, it very
quickly began to feed on other species and multiplied in numbers until it became a pest itself. Once
biological control agents are released, they cannot
be recalled and, as they are fertile, they have the
IAEA Bulletin 51-1 | September 2009 | 35
Animal Kingdom | To Kill a Pest
Improved Mass Production of
Biological Control Agents
♦ In Bulgaria, an irradiated factitious host
used to rear the wasp that parasitizes moth
pests in mills and grain warehouses.
♦ In Pakistan, irradiated moth eggs utilized
as a prey substitute to feed a predator for areawide control of cotton and sugarcane pests.
♦ In Poland, radiation used to successfully
extend the shelf-life of parasitoids to control
stored grain moths causing damage in grain
warehouses.
♦ In Turkey, an irradiated factitious host used
to mass rear a parasitoid of the olive fly for use
in an area-wide pilot project in an olive production area.
have to be reared in order to produce the biological
control agent for release, in other words it is a two
component biological system. This is in contrast to
the SIT where only one species needs to be reared.
This increased complexity makes the mass rearing
of biological control agents logistically demanding
and more expensive.
Top: Mucidifurax raptor
wasp on a fly puparium.
Once the female chooses
a suitable puparium host,
she lays a single egg in it.
The egg hatches, and the
wasp larva feeds on the
fly pupa.
(Photo: USDA ARS Photo
Unit, USDA Agricultural
Research Service,
Bugwood.org)
Bottom: Braconid wasps:
parasitoids on sphinx or
hawk moths.
(Photo: David Cappaert,
Michigan State University,
USA. Bugwood.org)
opportunity to reproduce and increase in numbers.
This is fine if they remain “locked onto” the pest species but can be disastrous if they find new non-pest
hosts in the new ecosystem.
Radiation can play an important role in the safe
evaluation of the potential host range of a biological control agent in a new ecosystem. It provides a
way to sterilize and release biological control agents
in the field without establishing them permanently
and without affecting their behaviour, and to assess
what they eat and do not eat, what hosts they parasitize, and where they go. Repeated releases of sterilized biological control agents will provide without
risk critical information under natural conditions in
order to improve decision making relating to eventual fertile release.
Improving Mass Production
As a biological control agent preys on or parasitizes
another insect species, it follows that both species
36 | IAEA Bulletin 51-1 | September 2009
Often the natural prey or host species are themselves difficult or expensive to rear in large numbers
and the use of more readily available substitutes
would be an advantage – the so called factitious
species. However, these types of species are not
always as acceptable as the natural prey or host; this
is especially the case for biological control agents
which lay eggs in living hosts which are then subject to its immune response. Radiation can be used
to suppress the immune response of the host making it more suitable for parasitism.
A host is often suitable for parasitism only within
a very small window of time during development
and radiation can be used to enlarge this window
by reducing the speed of development of the host.
The limited shelf life of hosts and prey also restricts
their use during mass-production and for certain
species radiation can be used to arrest development and thus allow for storage and stockpiling of
hosts or prey to be used when required by the customers (farmers, greenhouses, grain mills, poultry
houses, etc.).
Animal Kingdom | To Kill a Pest
There is also the use of the controversial phenomenon known as ‘radiation hormesis’, i.e. the use of
very low doses of radiation to stimulate biological
processes. There is some preliminary evidence that
this process can increase parasitisation rates and
reproduction.
Facilitating Handling,
Shipment, Trade and Release
A major headache for producers of biological control agents is the continued development and
emergence of some pest insects, in the form of nonparasitized hosts and unused prey insects, among
the mass produced natural enemies. This “contamination” of the final biocontrol product can create
major problems in terms of efficiency of the massproduction process. It requires additional handling
steps involving the removal of significant numbers
of non-parasitized hosts or unused prey individuals from the rearing process before they are shipped
and emerge as pest insects with farmers using the
biological control agents. Radiation can be used to
sterilize prey, hosts, and factitious hosts to prevent
further development of the pest insects and thus
remove the need for labour intensive separation
procedures.
When biological control agents are shipped to other
countries, the fact that shipments often include fertile pests, either as prey or hosts, brings with it a real
or perceived risk that this could lead to the introduction of non-native, pesticide resistant or new strains
of pest insects into new areas or countries. This risk
can be translated into ever more stringent quarantine regulations and permits required for their shipment. Irradiation of hosts and prey can ensure that,
even if not all the hosts are parasitized and all the
prey eaten, customers receive shipments free of fertile pest insects.
There is also the need to safely ship hosts and prey
between different facilities; for example large production facilities may decide to ship hosts/prey to
smaller satellite facilities which concentrate only
on rearing the biological control agent and not the
hosts or prey. This procedure can be made safe by
irradiating the material before shipment as is now
routinely done in SIT programmes, where sterile
pupae are shipped to large emergence and release
facilities. The use of radiation in this way will lead to
increased efficiencies in the production of biological control agents and will help to standardize the
use of strains of host/prey material to ensure product quality.
Supplementing Biological
Control Agents in the Field
♦ In China, irradiated moths released into field
crops where their sterile eggs served as hosts for
wild parasitoids resulting in parasitoid population increase.
♦ In Czech Republic, irradiated moth eggs distributed in a natural forest to serve as hosts for
wild biological control agents.
♦ In Czech Republic, sterile moth larvae
deployed in forests to monitor the density and
type of parasitoids and pathogens.
♦ In Pakistan, irradiated hosts placed in the
field early in the season to increase populations
of parasitoids to effectively manage sugarcane
pests in an area of 40,000 hectares.
Handling, Shipment,
Trade and Release of
Biological Control Agents
♦ In Argentina, radiation of housefly pupae
used to mass rear egg and pupal parasitoids for
deployment in chicken houses and cattle feedlots.
♦ In Mexico, irradiation of immature stages of
fruit flies to mass rear ca. 100 million fruit fly parasitoids each week, as part of the area-wide release
of the parasitoids.
♦ In the USA, irradiation of prey for the production of predatory mites for the control of vegetable pests in greenhouses.
Supplementing Biological
Control Agents in the Field
In the field, insect pests go through population cycles as do their biological control agents.
Unfortunately these cycles are often not in synchrony and the biological control agent populations generally lag behind the pest populations. If
the number of biological control agents could be
increased prior to the increase in the pest population then much better control would follow. This
can be achieved by distributing prey or hosts, sterilized by radiation, into the field early in the season so
IAEA Bulletin 51-1 | September 2009 | 37
Animal Kingdom | To Kill a Pest
a very difficult task as hosts can be rare or difficult
to locate or both. Radiation sterilized hosts can be
deployed in the field at strategic locations and so
increase the chances of collecting new biological
control agents.
Integrating Biological Control
with SIT
Pink spotted ladybird
predator feeding on
eggs of Colorado potato
beetle.
(Photo: Whitney
Cranshaw, Colorado State
University, Bugwood.org)
that the numbers of biological control agents can
be safely built up on the pest insects deployed.
Many years ago, the father of the SIT, E.F. Knipling,
suggested that it would be advantageous to combine the release of sterile insects with the release of
biological control agents. He suggested that a synergistic response in reducing the size of the target
population could be achieved as the sterile males
mate with the adult females in the wild assuring no
offspring, whilst the biological control agents target
the other developmental stages of the pest insect,
i.e. the egg, larva or pupal stages.
In biological control programmes it is necessary to
monitor the biological control agent in the field to
assess its population levels, survival, distribution
etc., and this again can prove to be quite difficult as
the numbers of hosts can be low in an effective programme. However, radiation sterilized hosts can be
safely introduced into the target location as sentinels to increase the chance of correctly evaluating
the presence and levels of the biological control
agent and so increase programme efficiency.
Jorge Hendrichs is Head of the IAEA’s Insect Pest Control
Section. E-mail: [email protected].
Exploring for, and collection of, new exotic biological control agents in countries of origin can be
Alan Robinson worked as a consultant in the same section. E-mail: [email protected].
Such an integration of sterile insects and other beneficial organisms has now been achieved in a number
of crop and pest situations, and there is great potential to expand this integrated and fully environmentfriendly biological approach. Integrating Biological Control Agents
and Sterile Insects
In Mexico, sterile fruit flies and parasitoids
released simultaneously as part of a large
national campaign that has eliminated
fruit flies from northwestern Mexico and
suppresses them effectively in other
areas.
In Syria, simultaneous releases of egg parasitoids and sterile moths synergistically
reduced field populations of the potato
tuber moth.
In South Africa, simultaneous releases of
egg parasitoids and sterile moths synergistically reduced field populations of
38 | IAEA Bulletin 51-1 | September 2009
the false codling moth in citrus orchards.
These findings have encouraged the
establishment of a private company by
the South African citrus expert industry.
In India, entomopathogenic nematodes
released together with irradiated moths
to control pests of cotton.
In Israel, by-products of insect mass-rearing used for the production of predators
of greenhouse pests and parasitoids of
houseflies.